The "hate" mainly comes from the fact that .40 S+W is an intentionally neutered cartridge. You can't goose it up, etc, as a result. It's an intentionally crippled 10mm. 10mm fans are pissed because it basically forced deprecation of the 10mm, which is the "real" .40.
As far as "knock down power" is concerned, you might want to look at wound ballistics. a little bit. The energy levels between 9mm, .40, and .45 are not huge orders of magnitude different from one another. It's all mostly user preference, really. Choose a good quality JHP in any of the above calibers and be done with it.
One reason I dislike the .40 is because idiots recommend it to newbs all the time, and that's about the dumbest piece of advice anyone can dispense. It's not that its really hard to shoot, but the awkward recoil impulse of many .40 S+W handguns takes some getting used to... and exposing a noob to that right off the bat makes it that much harder for a new person to learn how to shoot. This is part of the reason you see a large number of used .40 S+W handguns for sale at gun stores... people buy them, and because they lack training, etc, they end up not being able to hit the broad side of a barn with it, then they get frustrated and sell the gun. The same can happen with any gun, but I think the recoil of the .40 plays into flinching, etc, pretty hard with newbs.
The only good things going for the .40 right now is the ammo price and it keeps the grip circumference of the gun small... but even then, both of those goals are met pretty well by 9mm.
At one point I had 5 handguns chambered in .40 S+W.... but over time the caliber lost its usefulness to me and I sold them all. I might get another one someday but I have no immediate plans to.... It's too small for bowling pins, and too big (or I should say, overkill) for steel plates.... so the only use for it is really as a defensive caliber... and I have a bunch of other guns in .45 and 9mm that will do that just as well.... so having .40s around is "redundancy of the redundancy department" kind of stuff.
-Mike